In a Carrier Sense Multiple Access (CSMA) contention transmission mode, the nodes must continuously monitor the transmission channel either to send a message or to receive a message sent to them.
A sender node having data to send to a destination node sends the data only if the transmission channel is free. This makes it possible to limit collisions between data. Through continuous monitoring, the destination node for the data can detect and receive data that is intended for it.
In this continuous monitoring state, the nodes consume energy unnecessarily.
To alleviate this problem, some transmission modes are based on techniques that put nodes to sleep, for example the technique known as “preamble sampling”.
The nodes are asleep for a period of time that has the same duration for all the nodes. They wake up at the end of this period, possibly in an unsynchronized manner.
While it is awake, a node probes the transmission channel to detect transmitted data. If during the period in which it is awake it does not detect data intended for it, the node goes back to sleep. It wakes up again only after a period of sleep has elapsed.
That technique makes it possible to guarantee that a destination node detects data intended for it while it is awake.
To this end, the node sending the data precedes the data with a preamble. The duration of the preamble is at least as long as the period of sleep. Thus when the destination node for the data wakes up it receives the preamble.
Without specific refinement of that transmission mode, all nodes in the vicinity of the node sending data detect the preamble and remain awake until the data has been received. Analyzing the field containing the address of the destination node for the data makes it possible for nodes that are not the destination node to ignore it and go back to sleep.
In a more refined version of that transmission mode, it is possible to insert the address of the destination node into the preamble. Accordingly, when a node detects the preamble, it can tell whether it is the destination node for the data or not. If it is the destination node for the data, it then remains awake throughout the time needed to receive the data, until all the data has been received; if not, it goes back to sleep.
The time for which a node is awake is very short. It corresponds to the minimum time that a node needs in order to detect a preamble. This is very much less than the period of sleep. For example, for the ZigBee transmission mode (see IEEE standard 802.15.4), the duration of sleep is of the order of a few hundred milliseconds for a wakeful period of approximately thirty microseconds.
Using that technique, energy consumption is concentrated essentially at the sender nodes.
Some communications signals are subject to rules with which the nodes must comply. These rules may relate to the activity ratio of a node (the ratio between the sending time and the silence time over a given time period) or the transmission time of a node.
For example, in the European regulations relating to the transmission of ultrawideband (UWB) signals in a certain band of frequencies, the activity ratio is limited to 5% over a period of one second and 0.5% over a period of one hour and a node may not send continuously for more than five milliseconds.
The preamble sampling technique of putting nodes asleep has the drawback of not being applicable to transmission modes of the above type because the duration of the preamble may exceed the allowed sending time.